Processes for injection molding articles typically involve injecting a thermoplastic material into a mold with a cavity configured to form such an article. After allowing the thermoplastic material to solidify, the resulting article is removed from the mold. Prior to injection, the thermoplastic material is melted with heat and, after injection, is allowed to cool either by ambient conditions surrounding the mold or by providing the mold with passages through which a coolant circulates.
In molding articles with thermoplastic materials, it is often desirable to place an insert in the mold cavity and then inject thermoplastic material on or around the insert.
U.S. Pat. No. 3,351,691 to Wilford relates to a method of molding rocket nozzles by injecting a thermosetting material into a mold and around an insert member. In this process, material is injected into the mold below the level of the insert and allowed to flow radially outwardly and ultimately either downwardly away from the insert or upwardly around the insert.
In U.S. Pat. No. 4,115,506 to Shima, a ski is produced by injection molding where a thermoplastic resin containing glass fibers is injected into a mold over a ski body insert within the mold.
U.S. Pat. No. 4,021,524 to Grimsley relates to a method of making a collapsible tube with an integral cap by injecting a thermoplastic material into a mold containing the upper portion of a tubular body to which the closure is to be attached.
U.S. Pat. No. 4,430,285 to Runyan et al. forms a rotary plug valve housing in which the plug is positioned within the cavity of a mold, and molten polymeric material is introduced into the cavity around the plug.
U.S. Pat. No. 4,088,729 to Sherman relates to a method of forming the hub for an abrasive disk by placing the disk having a central opening in a mold cavity and then injecting thermoplastic material into the mold to form the centrally-located hub.
Other injection molding processes involving the use of an insert are disclosed in U.S. Pat. No. 4,689,190 to Peisker et al., U.S. Pat. No. 4,076,788 to Ditto, U.S. Pat. No. 4,040,670 to Williams, U.S. Pat. No. 3,268,643 to Katzenmeyer et al., U.S. Pat. No. 3,037,652 to Wallace, U.S. Pat. No. 4,017,961 to Kochte et al., U.S. Pat. No. 3,445,559 to Siteman, and U.S. Pat. No. 2,996,764 to Ross et al.
Although injection molding technology has been utilized to form a wide variety of articles from thermoplastic materials, it has not been utilized to produce mixing impellers due to the requirement that such devices have sufficient strength to overcome the forces applied against the impeller blades by the material being mixed. Instead, such impellers have been produced from metal or by building up layers of plastic material or by compression molding of resin containing comminuted glass fibers. Such techniques of forming mixing impellers, however, tend to be time consuming and expensive.